This invention relates to a method and apparatus for producing an ion stream from an electrospray formed from a sample solution and for analyzing the ion stream. More particularly this invention relates to an apparatus for receiving an electrospray and converting it to an ion stream for analysis.
A liquid flowing through a capillary jet or orifice may be converted to a spray of small charged droplets (of the order of 1 .mu.m in diameter) by applying a strong electric field to the liquid as it emerges from the tip of the capillary. For a sufficiently high applied field, the electrostatic stress imposed by the field and the surface-induced electrical charge is sufficient to overcome the surface tension forces on the liquid. Breaking apart into a large number of small charged droplets is a way for the liquid to disperse the charge and reach a lower total energy state. This process of forming a spray is commonly known as electrospray.
At the present time apparatus are available for forming an electrospray of a sample solution such as a liquid stream effluent from a liquid chromatography separation step and subsequently analyzing the electrospray with a mass analyzer such as a quadrupole mass spectrometer, an ion trap, a time-of-flight mass spectrometer or a magnetic sector mass spectrometer or the like. Interposed between the apparatus for forming the electrospray and the analytical apparatus is means for desolvating the droplets forming the electrospray so as to form a stream of intact ions which are to be analyzed. In a liquid chromatograph, a stream of solvent, containing a mixture of chemical species in solution, is passed at elevated pressure through a chromatographic column. The column is so designed that it separates the mixture, by differential retention on the column, into its component species. The different species then emerge from the column as distinct bands in the solvent stream, separated in time. Coupling the output of a liquid chromatograph to a mass spectrometer via an electrospray interface gives the analyst a powerful tool since it can provide molecular weight and structural information about the separated species as they emerge from the liquid chromatograph.
It has been proposed in U.S. Pat. No. 4,977,320 to interpose a heated capillary tube between the electrospray apparatus and the analytical apparatus to desolvate the electrospray droplets thereby to form the intact ion stream. The apparatus is undesirable since it is difficult to uniformly heat the thin-walled tube. This may result in local "hot spots" which can cause sample degradation. In addition, the capillary can become clogged due to sample deposition therein and it is difficult to clean for subsequent use. Furthermore, mechanical construction and alignment are difficult.
Allen and Vestal disclose in U.S. Pat. No. 4,999,493 (and in "Design and Performance of a Novel Electrospray Interface", Journal of the American Society of Mass Spectrometry 1992, 3, 18-26) the use of an electrospray interface that consists of three chambers interposed between the electrospray capillary and the analytical apparatus. A first spray chamber, a second skimmer chamber and finally a third heated skimmer chamber. It is diclosed that droplet desolvation and consequent ion generation occur in this heated chamber. In practice this apparatus is undesirable because to be effective the heated chamber must be typically at 200-250 C. This high heat level requires the spray chamber to be cooled by circulating water, which adds significantly to the complexity and operating cost of this interface.
Additionally, existing electrospray interfaces typically require the precise positioning of the electrospray capillary with respect to the remaining components of the interface in order to optimize sensitivity.
Accordingly, it would be desirable to provide a method and apparatus for efficiently converting an electrospray into an intact ion stream for subsequent analysis in a mass spectrometer or the like. In addition, it would be desirable to provide such an apparatus wherein the electrospray is substantially desolvated with uniform heating. Furthermore, it would be desirable to provide such an apparatus which can be cleaned, reassembled and realigned easily for subsequent reuse. Further desirable features of such an apparatus include not requiring a cooling means, not requiring precise positioning of the electrospray capillary with respect to the remaining interface components, not requiring an external supply of gas, and not requiring excessively large rotary vacuum pumps.